The present invention relates to continuously variable transmissions of various types such as a toroidal type and a V-belt type.
Japanese Patent Provisional Publication No. 63-130954 discloses a conventional toroidal type continuously variable transmission (CVT) having input and output cone discs forming a toroidal cavity therebetween, and two friction rollers which are compressed in the toroidal cavity between the input and output discs for torque transmission therebetween and which are inclined so as to continuously vary a transmission ratio of the output speed to the input speed.
Japanese Patent Provisional Publication No. 63-125852 discloses another conventional transmission system. This system has two parallel toroidal CVT units each of which has two friction rollers, and two actuators for actuating the two friction rollers, respectively. Similar toroidal type transmission systems are disclosed in the above-mentioned, commonly assigned, copending U.S. applications Ser. Nos. 07/313,418, 07/316,944 and 07/314,846, now U.S. Pat. Nos. 4,928,542, 4,968,289 and 4,893,517, respectively which are still kept in confidence.
Japanese Patent Provisional Publication Nos. 62-83553 and 63-34363 disclose V-belt type continuously variable transmission systems each having two parallel V-belt CVT units. Each of these conventional examples includes two actuators for actuating the two CVT units, respectively, and two shift control valves which are connected, respectively, with the two actuators.
However, these CVT systems having two drive units of the toroidal or V-belt type are still unsatisfactory. For example, an unbalance of torque between the two parallel CVT units readily results in an unbalance of the shift control between both units. Therefore, it is difficult to control the transmission ratio accurately. Especially the calculation of the actual speed ratio becomes more difficult as the rotational speed becomes lower (because of a lowering of the resolution).
The transmission system of the above-mentioned Japanese Publication No. 62-83553 is arranged to absorb the unbalance between the two parallel CVT units with a differential mechanism provided between both units. The addition of the differential mechanism, however, complicates the construction of the transmission system, and increases the size and weight of the system.